Heat engines transform heat into mechanical energy with the corresponding efficiency of an Otto engine amounting to only about 25 percent, which is what your automobile gets.

The efficiency of heat engines powered by thermal heat reservoirs is determined by the second law of thermodynamics, one of the fundamental concepts in physics. It was as far back as 1824 that Frenchman Nicolas Carnot calculated the maximum possible efficiency limit of such engines, now known as the Carnot limit.

Now imagine a heat engine that consists of just a single calcium ion. Such a nano-sized heat engine would be far more efficient than your car engine or a coal-fired power plant because we can theoretically exceed the classic Carnot limit by manipulating the heat baths and exploiting non-equlibrium states.

A prototype of such a single-ion heat engine is currently being constructed at Mainz University.

Simulation of an Otto cycle of a single ion heat engine: The enclosed area pictures the produced work that is significantly increased by way of squeezing. Credit: Quantum, Mainz University

Calculations and simulations from last year showed for the first time that the thermo-dynamic flow in an internal combustion engine could be reproduced using individual ions. The idea was to use a calcium 40 ion, which has a diameter a million times smaller than that of a human hair, for this purpose. "Individual ions can basically act as the piston and drive shaft or, in other words, represent the entire engine," explained Johannes Roßnagel of the Quantum, Atomic, and Neutron Physics (QUANTUM) work group of the JGU Institute of Physics.

Individual ions have already been captured in Paul traps and, using laser beams and electrical fields, not only cooled and heated but also compressed.

"This means we are able to manipulate the pulse location distribution for optimum efficiency," added Roßnagel. "Exceeding the Carnot limit for a standard heat engine thus does not violate the second law of thermodynamics but instead demonstrates that the use of specially prepared, non-thermal heat reservoirs also makes it possible to further improve efficiency."

In their article, the physicists calculated the general Carnot limit for this situation. As the mechanical capacity of a single ion machine is extremely low, it can probably only be used in heating or cooling nano systems.